Plastic articles with compatibilized barrier resin

ABSTRACT

This invention provides an improved adhesive layer for multi-layer films and bottles, the adhesive layer being a graft copolymer of a polyolefin backbone with a methyl methacrylate graft. The graft copolymer sufficiently improves the compatibility between barrier resins and polyolefins to permit the production of monolithic or single layer bottles comprised of a blend of those three components. The resultant products have improved physical properties while retaining acceptable permeability to gases.

This application is a Divisional application of U.S. Ser. No.07/912,080, filed Jul. 8, 1992, now U.S. Pat. No. 5,300,570, which is aContinuation application of U.S. Ser. No. 07/687,642 filed Apr. 19,1991, now abandoned, which is a Divisional application of U.S. Ser. No.07/413,943 filed Sep. 28, 1989, now U.S. Pat. No. 5,035,933, which is aContinuation-in-Part of U.S. Ser. No. 07/315,501, now U.S. Pat. No.4,957,974 filed Mar. 1, 1989, now U.S. Pat. No. 4,957,974 which is aContinuation-in-part application of application Ser. No. 07/174,648,filed on Mar. 29, 1988, now abandoned, the disclosure of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to improved extruded and molded articles, tofilms, bottles, and the like with improved impact strength and reducedoxygen permeability as a result of the incorporation of a compatibilizedbarrier resin into such articles, and to methods of preparing sucharticles.

2. Description of Related Art

The polyolefins combine advantageous properties both technically andcommercially. Polypropylene, for example, exhibits greater hardness andstiffness than polyethylene, lower brittleness than standardpolystyrene, outstanding resistance to hot water and to chemicals, andvery good electrical properties. Thus, the polyolefins and theircopolymers have a wide range of applications in the form of containers,moldings, profiles, tubes, films, fibrillated filaments and textilefibers.

Formed polyolefin articles may be prepared by injection or blow-molding.In particular, stretch blow-molding is a mass production process forplastic bottles replacing the glass variety. These are used for oil,wine, spirits, milk, still and CO2-containing mineral water, soft drinksand beer. Although the polyolefins possess many desirable properties, asenumerated above, in the particular applications of containers andpackaging films, it is desirable to reduce the gas permeability belowthat normally exhibited by polyolefin films and bottles.

Typically, an extruded film or blown bottle will be comprised of severallayers of different polymers, arranged to exploit desirable propertiesand compensate for detrimental properties. For example, polypropyleneoffers structural integrity, but is permeable to many gases. A layer ofa gas barrier resin, such as a nitrile barrier resin, will reduce thepermeability to acceptable levels, even though the barrier resin itselfmay have undesirable properties, such as sensitivity to liquids. Thebarrier resin may be sandwiched between layers of the structuralpolyolefin to isolate the resin, while obtaining the advantages offeredby the barrier. In particular, copolymers of ethylene with vinyl alcohol(EVOH) are used in the co-extrusion of packaging materials on account ofthe outstanding barrier properties against oxygen, nitrogen, carbondioxide and fragrances imparted by the EVOH. Other polymers, such aspoly(vinylidene chloride) (PVDC) may be used as the barrier resin. Theselection of barrier resin is well within the purview of the skilledartisan and is not limited to EVOH and PVDC.

One means for the preparation of such desirable packaging materials,such as films and bottles, is the co-extrusion of an outer layer ofpolyolefin, a layer of barrier resin, and an inner layer of polyolefin.

However, the adhesion between the polyolefin and barrier layers isgenerally considered to be inadequate for most uses. Therefore, it isnecessary to use an adhesive or tie layer between the polyolefin and thebarrier layers in order to ensure sufficient mechanical strength toprevent delamination. Typical adhesive materials are copolymers ofethylene, such as Plexar (Quantum Chemical, USI Division).

A representative five layer bottle may then have an outer polypropylenelayer, an adhesive layer, an EVOH barrier layer, a second adhesivelayer, and an internal polypropylene layer. Typically, the percentage ofadhesive will be about 10% by thickness. A representative constructionis approximately 14 mil polypropylene, 1.5 mil adhesive, 4 mil EVOH, 1.5mil adhesive, and 14 mil polypropylene.

In extrusion blow-molding of bottles of this type, excess material mustusually be removed from the finished product. This "scrap" will containpolypropylene, EVOH, and adhesive. For practical purposes, it isdesirable to co-extrude this scrap layer with virgin polypropylene,adhesive, and barrier resin forming a six-layer container in which thescrap layer is sandwiched between the outer polypropylene layer and thefirst tie layer. In this way, the losses attributable to scrap aresignificantly reduced. This co-extrusion, however, often leads tosignificant problems with delamination and product quality.

Since it is a critical element of these constructions to have thebarrier layer, it is also important that the barrier layer be one whichis state-of-the-art in terms of its properties. Although EVOH is awidely used and highly regarded material, it suffers from the deficiencyof being susceptible to water. The presence of the polypropyleneprotects it from water. However, in the construction of these bottlesand films, it is still necessary to use the adhesive. The adhesivematerial adds increased cost and raises significant questions ofprocessibility. It, therefore, is desirable to identify additionaladhesive materials which will provide a better balance of propertiesthan that currently available at improved processing efficiencies.

Summary of the Invention

This invention provides an improved adhesive layer for multi-layer filmsand bottles, the adhesive layer being a graft copolymer of a polyolefinbackbone with a methyl methacrylate graft. In addition, it has beendiscovered that the graft copolymer sufficiently improves thecompatibility between barrier resins and polyolefins to permit theproduction of monolithic or single layer bottles comprised of a blend ofthose three components. The resultant products have improved physicalproperties while retaining acceptable permeability to gases.

DETAILED DESCRIPTION OF THE INVENTION

The graft copolymer which serves as the adhesive or compatibilizingresin for this invention is disclosed in U.S. Ser. No. 07/315,501, filedMar. 1, 1989, now U.S. Pat. No. 4,957,974 of common ownership with thisapplication. The disclosure of the aforementioned application isincorporated by reference herein.

The graft polymer is derived from at least about 80% of a monomer of amethacrylic ester of the formula CH₂ ═C(CH₃)COOR, where R may be alkyl,aryl, substituted or unsubstituted, and less than 20% based on the totalmonomer weight, of an acrylic or styrenic monomer copolymerizable withthe methacrylic ester. This is accomplished by adding the methacrylatemonomers to a solution of the polyolefin together with an initiatorwhich generates a constant, low radical concentration, or radical"flux," at the solution temperature. These radicals initiatepolymerization of the monomer and cause formation of a covalent bondwith the trunk.

The molecular weight of the polyolefin polymer which forms the trunk ofthe graft copolymer should be high enough to give a large amount ofnon-polar polymer when grafted, but low enough so that most of the graftcopolymer has one acrylic polymer chain grafted to each polyolefin trunkchain. The trunk may have a molecular weight between 50,000 and1,000,000. The trunk may also have a molecular weight of about 100,000to 400,000. A polyolefin trunk having a molecular weight of about200,000-800,000 Mw is especially preferred, but polyolefins having amolecular weight of about 50,000-200,00 can be used with some beneficialeffect.

The preferred monomer is methyl methacrylate. As much as 100% of this,or of other 2 to 4 carbon alkyl methacrylates, can be used. Up to 20% ofhigh alkyl, such as dodecyl and the like, aryl, such as phenyl and thelike, alkaryl, and such as benzyl and the like, and/or cycloalkyl, suchas cyclohexyl and the like, methacrylates can be used. In addition, upto 20% (preferably less than 10%) of the following monomers can beincorporated with the methacrylate esters which form the major portionof the monomer: methacrylic acid, methacrylamide, hydroxyethylmethacrylate, hydroxypropyl methacrylate, alkoxyalkyl methacrylates,such as ethoxyethyl methacrylate and the like, methacrylamide,t-butylaminoethyl methacrylate, dimethylaminoethyl methacrylate,dimethylaminopropyl methacrylamide, glycidyl methacrylate,methacryloxypropyltriethoxysilane, acrylate monomers (such as ethylacrylate, butyl acrylate and the like), styrene, acrylonitrile,acrylamide, acrylic acid, acryloxypropionic acid, vinyl pyridine, andN-vinylpyrrolidone. In addition, as much as 5% of maleic anhydride oritaconic acid may be used. It is important that the chain transfer ofthe polymerizing chains to its own polymer be minimal relative totransfer with the polyolefins chains for the efficient production ofhomogenous non-gelled graft polymer in good yield.

The molecular weight of the acrylic graft as measured by the weightaverage molecular weight of the ungrafted co-prepared acrylic polymermay be about 20,000 to 200,000. The preferred range is 30,000 to150,000.

The process of graft polymerizing the monomer leads to the production ofungrafted and grafted material. The amount of grafted material is in therange of 5% to 50% of the total acrylic polymer or copolymer produced.The graft copolymer is prepared in a process that polymerizes themonomer in the presence of the non-polar polyolefin. The process isconducted in a solvent which swells or dissolves the non-polar polymer.The solvent is also one that has no or low chain transfer ability.Examples include non-branched and branched aliphatic hydrocarbons,chlorobenzene, benzene, t-butylbenzene, anisole, cyclohexane, naphthas,and dibutyl ether. Preferably, the solvent is easy to remove byextrusion devolalization, and therefore has a boiling point below 200°C., preferably below about 150° C. To avoid excessive pressure, aboiling point above about 100° C. is also preferred.

The final solids content (which includes polyolefin and acrylic polymer)depends on the viscosity and the ability to mix well. The practicallimits are 20% to 70% but the solids content can be as high as itsconsistent with good mixing for economy. Preferably, the solids contentfalls in the range of about 35% to about 60%.

A gradual addition or multicharge addition of the monomer is preferred.Optionally, the monomer charge need not be the same throughout, forexample, the last 0-20% may contain all of the monomer used in minoramount to concentrate that monomer in one portion of the polymer.

The temperature during the polymerization can be in the range 110° C. to200° C., but the preferred range is 130° C. to 175° C. Especiallypreferred is 145° C. to 160° C. The pressure can be atmospheric tosuperatmospheric, or as high as 2100 kPa or whatever is necessary tokeep the reaction mixture in the liquid phase at the polymerizationtemperature.

The unreacted monomer concentration should be kept low during thereaction. This is controlled by balancing the radical flux and themonomer feed conditions.

This application teaches the preparation of a variety of graftcopolymers of polyolefins, in particular polypropylene, withmethacrylate side chains. The use of this copolymer in a variety ofresin systems has been seen to provide vastly improved compatibility, asevidenced by reduced domain sizes in polymeric blends. This applicationdiscloses and claims a significant advance in the film- andbottle-making art based upon the use of this novel copolymer as acompatibilizer for known resins.

EXAMPLE 1 Preparation of Acrylic/Polypropylene Graft CopolymerCompatibilizer

A polypropylene-acrylic graft copolymer was made by polymerizing a 5%ethyl acrylate-95% methyl methacrylate monomer mixture in the presenceof polypropylene (weight ratio of polypropylene:monomer=0.67:1).Radicals were generated from di-t-butyl peroxide at the rate of 0.000070moles/liter/minute (radical flux). Monomer and initiator were fed over120 minutes and the theoretical (100% conversion) solids at the end ofthe reaction is 50%.

A 100-gallon reactor equipped with a pitched blade turbine agitator wascharged with 190 lb. of Isopar E (a mixed aliphatic hydrocarbon solvent)and 76 lb. of polypropylene (Himont 6523, Himont, Inc., Wilmington,Delaware). This mix was deoxygenated by applying vacuum to degas,followed by pressurizing with nitrogen to atmospheric pressure for threecycles. Finally, the mix was pressured to 15 psig with nitrogen andheated to 150° C. over 2 hours. A pressure of 35 psig was maintainedwhile the batch was held at 150° C. for 3 hours. Two solutions wereadded over a 15-minute period. The first consisted of 59 g of di-t-butylperoxide in 841 g of Isopar E. The second consisted of 0.32 kg of ethylacrylate and 6.14 kg of methyl methacrylate. An additional 103 g ofdi-t-butyl peroxide and 1479 g of Isopar E were added over 105. minutes.At the same time, 2.26 kg of ethyl acrylate and 43.0 kg of methylmethacrylate were added over 105 minutes. The reaction exothermincreased the temperature to about 160° C. After the feed was complete,11 lb. of Isopar E was fed into the reaction mixture.

The reaction mixture was held in the reaction kettle for an additional30 minutes. It was then transferred to a second kettle, which was alsounder pressure at 150° C. During the transfer, a solution of 80 g ofdi-t-dodecyl disulfide in 320 g of Isopar E was added to the secondkettle. Also during this transfer, three 10 lb. batches of Isopar E werefed into the reaction kettle. The material in this second kettle was fedto a 0.8 inch twin screw extruder, where devolatilization occurred.

During the devolatilization, the next batch was prepared in a reactionkettle. It was transferred to the extruder feed kettle while extrusioncontinued. In this way, several batches were made in a "semi-batch"matter, batch-wise in the reactor with continuous feed to the extruder.

Three samples of this material isolated at different times-during theextrusion were blended with Himont 6523 polypropylene in a ratio of4:96, pressed and tested for sag as described in U.S. Pat. No.4,957,974. All three gave properties within the range of that expectedfor acceptable material.

A second batch of graft copolymer was prepared in a similar manner,except that the radical flux was 0.000050 (42 g of di-t-butyl peroxideplus 858 g of Isopar E in first feed; 73 g of di-t-butyl peroxide and1502 g of Isopar E in second feed). Measurements of properties on thisbatch were also within the expected ranges.

The graft copolymer (GCP) used for the experiments described in thefollowing examples was prepared by blending pellets from thirteenbatches prepared in the first run and one batch of material prepared inthe second run.

EXAMPLE 2 Preparation of Melt Blends of Polypropylene, EVOH, and GraftCopolymer (GCP)

Polypropylene, EVOH and the graft copolymer of Example 1 were compoundedin an intermeshing, co-rotating twin screw extruder (Baker-Perkins,MPC/V 30) with an L/D of 10:1. The compounder was run at 200 rpm with amelt temperature of 205°-225° C. The melt was fed directly to a 38 mmsingle screw pelletizing extruder with an L/D of 8:1. The melttemperature in the transition zone between the compounding and thepelletizing extruder was 200°-215° C. The melt was stranded through adie, cooled in a water bath, and chopped into pellets. Visualobservation indicated that the materials containing graft copolymer werefully compatible. These pellets were then dried and injection molded ona reciprocating screw injection molding machine (New Britain Model 75)into test specimens.

ASTM test methods were used to test the impact strength and tensileproperties of the injection-molded parts. The results are summarized inTable 1.

EXAMPLE 3 Preparation of Films from polypropylene/EVOH/GCP Blends

Five mil (0.13 mm) films were pressed for determination of permeability.The pellets that had been compounded as described in Example 2 weremilled on a 3 inch by 7 inch electric mill at 205° C. for 3 minutes. Thehot stock was then placed between preheated plates and pressed at 205°C. in a 100-ton Farrel press for 3 minutes at 2 tons followed by 2minutes at 20 tons. The plates were transferred to a cool press for 1minute of cooling at 2 tons pressure.

Oxygen permeation values were determined using a Mocon Ox-Tran 1000tester (Modern Controls, Inc., Brooklyn Center, Minn.). The films fortesting were prepared as 110 mm squares sealed into the unit and sweptwith nitrogen on both sides to determine a sample baseline and allow thefilm to equilibrate with nitrogen.

Pure oxygen at 1 atmosphere pressure was then swept over one face of thefilm for the duration of the test. The nitrogen swept over the oppositeface of the film contained 1-2% hydrogen; this gas mixture was conductedfrom the test chamber through a Coulox nickel cadmium fuel celldetector, where any oxygen present burnt an equivalent amount of theexcess hydrogen to generate an electric current proportionate to theamount of oxygen. This current, automatically corrected for the samplebaseline, was continuously recorded and used to calculate the oxygenpermeability value of the sample.

Test conditions during both equilibration and oxygen testing were 23° C.and 0% relative humidity.

The permeability data are reported in Table 1 incc.cm/(sq.cm.cm.Hg.sec)xE(*12), where cc is cubic centimeters of oxygen,cm is film thickness in centimeters, sq cm is area of film in squarecentimeters, cm Hg is pressure in cm of Hg, sec is time in seconds, andE(*12) is 10 to the 12th power.

The addition of 5-15% of the graft copolymer compatibilizer to binarycompositions of EVOH and polypropylene improves the compatibility of thepolar EVOH and the non-polar olefinic polypropylene.

The first three entries in Table 1 indicate that the compatibilizingcopolymer does not negatively affect the permeability of EVOH films.Comparison of tests 4 and 5 indicates that the addition of approximately5% of the GCP compatibilizer almost doubles the unnotched Izod impactstrength, without affecting the permeability, and further increases themaximum stress, break strain, and tensile modulus.

When polypropylene is the majority component, it can be seen from test 7that the oxygen permeability is rather high. The addition of the GCP atapproximately 5% in test 8 reduces the permeability by 33% whileenhancing the physical properties. Even with the very high percentagesof polypropylene in tests 10, 11 and 12, it can be seen that thepresence of the compatibilizing additive improves the physicalproperties.

EXAMPLE 4 Preparation of a Co-extruded Five-Layer Bottle Using theCompatibilizing Additive

A five-layer bottle was extruded through five separate BattenfeldFischer extruders. Each extruder is independently controlled. Two moldsfor continuous extrusion of the parison were present. The materials usedwere polypropylene (Fortilene 41X11, Soltex), a layer of Plexar 460adhesive (Quantum Chemical, USI Division), a barrier layer of EVOH (EVALSC-F101, Eval Corporation of America), a second Plexar adhesive layer,and a final inner polypropylene (Fortilene 41X11) layer.

The polypropylene was extruded at a temperature between 190° and 200° C.The EVOH was extruded at a temperature between 205° and 210° C. In thefirst trials, Plexar 460 was used as the adhesive and was maintained ata temperature between 200° and 220° C.

An acceptable five-layer bottle was produced in these experiments.

The graft copolymeric compatibilizer(GCP) of Example 1 was substitutedfor both layers of Plexar 460 and it was determined that a GCP melttemperature of only 172° C. was needed to extrude coherent bottles whichwould withstand delamination using this new combination.

EXAMPLE 5 Preparation of a Monolithic Bottle from a Blend ofPolypropylene, EVOH, and GCP Compatibilizer

A monolithic, single-layer bottle was prepared by extrusion blow-moldingat about 200° C. of a blend of polypropylene, EVOH, and thecompatibilizing additive of Example 1. The actual material used for thisexperiment was reground scrap from Example 4. Although this bottle washazier than the traditional layered construction described previously,it was uniform in appearance and also did not have delaminationproblems. It is expected that this monolithic bottle should be useful inmany applications in which permeability, strength, and optical clarityare desired to be between those of polypropylene and a multi-layeredbarrier construction.

                                      TABLE 1                                     __________________________________________________________________________    PHYSICAL PROPERTIES OF EVOH/POLYPROPYLENE/GRAFT                               COPOLYMER BLENDS                                                                 EVOH/PP/GCP                                                                            Maximum                                                                              Tensile Break Unnotched                                                                           Oxygen                                 NO.                                                                              (%)      Stress (Kpsi)                                                                        Modulus (Kpsi)                                                                        Strain (%)                                                                          Izod (ft-lb)                                                                        Permeability                           __________________________________________________________________________    1  100/0/0  8.6    479     114   --     0.05                                  2  95/0/5   8.5    466     99    --     0.08                                  3  87/0/13  8.1    454     11    --     0.05                                  4  70/30/0  5.9    380      6    6.9    0.09                                  5  67/29/5  6.8    394     16    13.0   0.10                                  6  61/26/13 6.8    387      8    6.7    0.12                                  7  45/55/0  4.9    324      4    4.8   17.50                                  8  43/52/5  5.8    334      7    6.5   11.70                                  9  39/48/13 5.9    337      8    5.7   --                                     10 20/80/0  4.8    279     10    4.8   14.91                                  11 19/76/5  5.2    296     29    7.4   14.97                                  12 17/70/13 5.4    306     12    8.5   13.84                                  13 0/100/0  4.6    224     100   20.0  97.50                                  14 0/95/5   4.8    248     100   20.0  --                                     15  0/87/13 4.9    262     100   21.0  --                                     __________________________________________________________________________

While the invention has been described with reference to specificexamples and applications, other modifications and uses for theinvention will be apparent to those skilled in the art without departingfrom the spirit and scope of the invention defined in the followingclaims.

What is claimed is:
 1. A polymeric blend of a gas barrier resin and apolyolefin-methacrylate graft copolymer, wherein said graftcomprises:(a) a non-polar polyolefin trunk selected from the groupconsisting of polyethylene, polypropylene, polybutylene,poly(4-methylpentene), copolymers of said olefins with each other, andone or more copolymers of said olefins with minor amounts of 1-alkenes,vinyl esters, vinyl chloride, methacrylic ester, and methacrylic acid,said trunk having a molecular weight, M_(w), of between about 50,000 andabout 1,000,000; and (b) at least one methacrylate chain grafted with acovalent bond to said trunk, having a weight ratio with said trunk offrom about 1:9 to about 4:1, said chain being a polymer derived from atleast about 80 % of a monomer of methacrylic ester of the formula

    CH.sub.2 ═C(CH.sub.3)COOR,

where R is alkyl, aryl-substituted alkyl, aryl, or alkaryl, and lessthan about 20% or an acrylic or styrenic monomer copolymerizable withthe methacrylic ester, said chain having a molecular weight, M_(w), offrom about 20,000 to about 2000,000, wherein the gas barrier resin is acopolymer of ethylene and vinyl alcohol.
 2. A n-monolithic bottle withlow permeability to gases, comprising a blend of:(a) a polyolefin whichis polypropylene; (b) a gas-barrier resin which is a copolymer ofethylene and vinyl alcohol; (c) a polyolefin-acrylic graft copolymercompatibilizer for the barrier resin and polyolefin, in which the graftcopolymer compatibilizer comprises:(1) a non-polar polyolefin trunkconsisting of polypropylene, said trunk having a molecular weight,M_(w), of between about 50,000 and about 1,000,000; and (2) at least onemethacrylate chain grafted with a covalent bond to said trunk, having aweight ratio with said trunk of from about 1:9 to about 4:1, said chainbeing a polymer derived from at least about 80% of a monomer ofmethacrylic ester of the formula

    CH.sub.2 ═C(CH.sub.3)COOR,

where R is alkyl, aryl-substituted alkyl, aryl, or alkaryl, and lessthan about 20% or an acrylic or styrenic monomer copolymerizable withthe methacrylic ester, said chain having a molecular weight, M_(w), offrom about 20,000 to about 2000,000.
 3. The monolithic bottle of claim2, comprising about 4 to about 10% by weight graft copolymer.